Friction stir welding is currently being used in many industries for the joining of metallic materials. One use for friction stir welding is to weld a joint between overlapping sheets of metallic material. With reference to FIG. 1, a conventional friction stir welding tool 10 is schematically shown including a body 12 having an integral tip 14 extending therefrom. Both the body 12 and tip 14 are together rotatable about an axis for friction stir welding a joint between two sheets 16, 18 of overlapping metallic sheet material, such as two sheets of steel, two sheets of aluminum, one sheet of steel and one sheet of aluminum, etc. Problems associated with such conventional friction stir welding tools include controlling local heating of the sheets 16, 18 and providing uniform heating at the joint between the sheets 16, 18.
To address these concerns, static shoulder friction stir weld tools were developed. In particular, the development of static shoulder friction tools was out of a need to provide more uniform heating through the thickness, improve surface quality and improve weld quality in friction stir welds. With reference to FIG. 2, a friction stir welding tool 20 is schematically shown including a rotating pin 22 disposed in a non-rotating or static body 24. The static body 24 includes a static shoulder 26 that slides across the surface 16a of the top sheet 16, while the rotating pin 22 plunges and spins into the sheet 16.
FIGS. 3 and 4 show a relative comparison between a friction stir welding joint using the friction stir welding tool 10 of FIG. 1 and the friction stir welding tool 20 of FIG. 2. In particular, FIG. 3 shows a weld joint 30 created by the tool 10 of FIG. 1 and FIG. 4 shows a weld joint 32 created by the tool 20 of FIG. 2. The comparison of FIG. 3 versus FIG. 4 shows that the tool 20 having the static shoulder 26 reduces the overall size of the heat affected zone (HAZ), which is predicted to better contribute to retaining base substrate properties. An additional benefit is that the mixing occurs uniformly from the shoulder 26 down through the depth of plunge of the pin 22 in the joint shown in FIG. 4. However, one drawback of the tool 20 is that while providing preferred mixing, the same mixing characteristic has been identified as creating an upward material flow along the pin 22 which forces highly plasticized, heated material (e.g., aluminum) into and beyond the pin bearing 34, which can foul the internal hardware of the tool 20 and/or cause significant maintenance issues for the tool 20.
In response to this challenging flow path, with reference to FIG. 5, pin geometry has been modified to limit mixing only in the overlap area between the sheets 16, 18. In particular, the tool 20′ of FIG. 5 includes a pin 22′ having a fluted end 22a′. While the fluted end 22a′ does reduce or eliminate the material flowing upwards into the tool 20′, this arrangement does not allow for uniform mixing near the surface 16a of the sheet 16. More particularly, the resulting joint between the sheets 16, 18 created by the tool 20′ tends to have connectivity or weld defects (e.g., inclusion of an undesirable hook feature 36, non-uniform mixing, etc.). This is best shown FIGS. 6 and 7. In particular, shown in FIG. 6, non-uniform mixing occurs due to the fluted end 22′, which mixes the middle lower half of the material. The undesirable hook 36 provides the initiation point of tensile failure for the joint between the sheets 16, 18 as best illustrated in FIG. 7.